5 Comparison with other clusters in similar directions

The present sample of 7 clusters belongs to the young disk. Although small, it is interesting to compare it with open clusters already studied in the literature in similar disk zones. This is because the present sample deals with relatively faint clusters, on the average fainter than more prominent objects which have had CMD studies in the literature. We must recall that clusters like the present ones are numerous in catalogs and will surely be the most frequent kind of young disk targets for CMD studies in coming years.

The WEBDA database (Mermilliod 1999) keeps updated parameters from studies in the literature. We selected all young disk clusters (about 600 Myr or younger, we recall that Hyades age clusters or older characterizing the old disk have their own properties and observational requirements) in 90$^\circ$ sectors centered respectively at $l = 270^\circ$ and towards the Galactic center ( $l = 0^\circ$). There are respectively 163 and 130 clusters, while our sample includes respectively 3 and 4 objects in such zones.

We show in Figs. 8 and 9 the reddening histograms in the two sectors respectively, and in Figs. 10 and 11 the age histograms respectively. The properties of the present clusters are indicated by arrows. Reddening and especially age distributions in both sectors are similar. Although relatively faint, only Hogg15 and Pismis21 are highly reddened in our sample, and they are also among the most reddened in the sectors. The frequency decrease of clusters with increasing age is remarkable. The distribution must reflect the formation/dissolution rates of star clusters (Wielen 1971, 1988), perhaps biased by observational constraints. However, what our small faint sample suggests is that the distribution will not change much in the future when new faint clusters are observed. Only one cluster (Lyngå11) occurs for larger ages. Since an observational bias appears improbable, the age histograms must intrinsically indicate a real drop. Unless major star forming events had occurred in the Galactic disk in the last 100 Myr or so, the results would favour an important dissolution rate of star clusters.